The present invention relates to a detection medium used for simultaneously detecting gross and fine leaks in hermetic seals.
Hermetic seals are used in a wide variety of applications. For example, in the electronics industry, solid state devices must be protected from the ambient atmosphere to guarantee their continued operation. Ambient air containing moisture can accumulate in the device causing corrosion and failure. High reliability devices are often protected by enclosing the devices in ceramic packages which are hermetically sealed. However, it is not possible to obtain a zero leak rate for every package. The packages must be tested to determine if the leak rate is below a set standard for a given internal sealed volume.
The most common standard employed for ceramic packages is provided in Military Specification ("Mil. Spec.") 883D (previously 883C), Method 1014.9. Standard leak rates are based on the leak rate of dry air at 25.degree. C. flowing through a leak path with a high pressure side of the leak at 1 atmosphere (760 torr absolute) and a low pressure side of the leak at less than 1 torr absolute.
Hermetic seal testing of ceramic packaging has traditionally been performed in two steps. The first step is designed to expose fine leak rates of 1.times.10.sup.-5 atm-cc/sec or less of dry air. A detectable gas is used to penetrate the fine leak openings. The second step is designed to expose gross leak rates between 1.times.10.sup.0 atm-cc/sec and 1.times.10.sup.-5 atm-cc/sec of dry air. A liquid is used to penetrate the gross leak openings. The fine leak test fails to detect gross leak openings because the gas diffuses too rapidly from the package to be detected. Historically, the fine leak test has been performed before the gross leak test because of a common belief that the liquid may block or even close a fine leak, thereby preventing its detection. Also, it is specified in Mil. Spec. 883D that fine leak testing should occur first.
Fine leak testing is typically performed using a tracer gas of either helium or krypton-85. Helium leak detection is the most common way to measure fine leaks. The measurement consists of placing a package to be tested into pressurized helium to force helium through the leaks and into a cavity in the package. The pressure level and duration of the pressurization step are defined in Mil. Spec. 883D, Method 1014.9. The package is removed from this environment and then tested for the presence of helium. Any helium escaping from the package cavity is detected with a helium mass-spectrometer leak detector. The helium leak test is very sensitive and is reported to detect leaks down to 1.times.10.sup.-12 atm-cc/sec.
The krypton leak test is another method of detecting fine leaks. The package to be tested is placed into an atmosphere of radioactive tracer gas containing a mixture of krypton-85 and dry nitrogen. The atmosphere is pressurized according to Mil. Spec. 883D, Method 1014.9. If a leak exists, the radioactive tracer gas will be forced through the leak opening and into the package. Faulty seals can be detected by measuring the radioactive decay from within the package cavity with a scintillation crystal-equipped counting station after the package is removed from the radioactive environment. Background radiation is compared to the measured value to determine the magnitude of the leak.
Gross leak testing involves using either fluorocarbon liquids or a dye penetrant, as provided in Mil. Spec. 883D. The dye penetrant leak measurement involves detecting fluorescent dyes which have entered the package. The fluorocarbon liquid tests include a bubble test, a weight gain test, and a negative ion detector (NID) test. These tests are non-destructive.
The bubble test uses fluorocarbon detector liquids such as FLUORINERT.TM. FC-72.TM. and FC-84.TM., which are manufactured by 3M, or PP-1.TM. which is manufactured by the Imperial Smelting Corporation. The package to be tested is placed in a "bombing chamber." The detector liquid is "bombed" into the leaky package under a pressure of up to 90 psia (0.62 MPa) for up to 12.5 hours. After bombing, the packages are removed and dried.
The packages are then placed into a bubble tank for leak detection. The bubble tank contains a fluorocarbon indicator liquid, such as FLUORINERT.TM. FC-40.TM. and FC-43.TM., which are manufactured by 3M, or PP-7.TM. and PP-9.TM. which are manufactured by the Imperial Smelting Corporation. The indicator liquid is heated to about 125.degree. C..+-.5.degree. C. The packages are immersed into the indicator liquid to a minimum depth of about two inches. If there is a leak in the package, the internal pressure within the package causes bubbles to form. The formation and size of the bubbles are monitored against a lighted, flat black background. If no bubbles form within a 30 second period, the package is considered to have no gross leaks.
The gross leak bubble test is reported to suffer from operator subjectivity and blocking of leaks by particulate. The bubble test does, however, provide a low cost and relatively fast test which is currently in wide use in production.
The weight gain test is another gross leak test which is commonly used. The weight gain test is described in Mil. Spec. 883D, Method 1014.9, Test Condition E. The weight gain test detects a leak by measuring the change in weight of a package after fluorocarbon liquid has been forced into the package through the leak. Packages to be tested are cleaned, dried and weighed. The packages are then grouped into "cells" depending upon their internal volume. Packages with an internal volume of less than 0.01 cc are put into cells of 0.5 milligram increments and packages with an internal volume greater than or equal to 0.01 cc are put into cells in 1 milligram increments.
The packages are placed under a 5 torr vacuum for one hour. A fluorocarbon liquid, such as FC-72.TM. manufactured by 3M or another equivalent liquid, is admitted into the bombing chamber to cover the packages without breaking the vacuum. The packages are pressurized, for example, to 75 psia (0.52 MPa) for two hours. For sensitive parts, a lower pressure may be used with a longer bombing cycle. After bombing, the parts are air dried for approximately two minutes.
The packages are weighed individually and categorized. A package is rejected as a leaker if it gains 1.0 milligrams or more. When the packages are categorized, any package which shifts by more than one cell shall be considered a reject. If a package loses weight, it may be retested after baking for eight hours at 125.degree. C. The weight gain test is more accurate and less operator subjective than the bubble test. However, the weight gain test is considered to be labor intensive and expensive to complete because of the sensitive balances required.
The "NID" test is described in Etess U.S. Pat. No. 4,920,785. The NID test was developed by Web Technology as an attempt to automate the weight gain test. The amount of fluorocarbon material evolving from the package after the bombing step is measured by measuring the infrared absorption of the atmosphere from the test chamber. The measured amount is proportional to the gross leak size.
Etess suggests other measurement instruments can be used with the NID test procedure. These instruments include an ultraviolet spectrometer, a thermal conductivity detector, a photoionization detector and an electron capture detector. However, the detector system manufactured by Web Technology employs an infrared absorption detector. It is believed that this system is currently not sensitive enough to detect fine leaks.
Performing separate gross and fine leak tests for each package is very time consuming and significantly adds to the cost of producing a product.